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+/*
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+ * Copyright (c) 1992, 1993
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+ * The Regents of the University of California. All rights reserved.
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+ *
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+ * Redistribution and use in source and binary forms, with or without
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+ * modification, are permitted provided that the following conditions
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+ * are met:
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+ * 1. Redistributions of source code must retain the above copyright
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+ * notice, this list of conditions and the following disclaimer.
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+ * 2. Redistributions in binary form must reproduce the above copyright
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+ * notice, this list of conditions and the following disclaimer in the
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+ * documentation and/or other materials provided with the distribution.
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+ * 3. All advertising materials mentioning features or use of this software
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+ * must display the following acknowledgement:
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+ * This product includes software developed by the University of
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+ * California, Berkeley and its contributors.
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+ * 4. Neither the name of the University nor the names of its contributors
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+ * may be used to endorse or promote products derived from this software
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+ * without specific prior written permission.
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+ *
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+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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+ * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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+ * SUCH DAMAGE.
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+ */
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+
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+/* @(#)log.c 8.2 (Berkeley) 11/30/93 */
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+#include <sys/cdefs.h>
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+__FBSDID("$FreeBSD: src/lib/msun/bsdsrc/b_log.c,v 1.9 2008/02/22 02:26:51 das Exp $");
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+
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+#include <math.h>
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+#include <errno.h>
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+
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+#include "mathimpl.h"
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+
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+/* Table-driven natural logarithm.
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+ *
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+ * This code was derived, with minor modifications, from:
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+ * Peter Tang, "Table-Driven Implementation of the
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+ * Logarithm in IEEE Floating-Point arithmetic." ACM Trans.
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+ * Math Software, vol 16. no 4, pp 378-400, Dec 1990).
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+ *
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+ * Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256,
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+ * where F = j/128 for j an integer in [0, 128].
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+ *
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+ * log(2^m) = log2_hi*m + log2_tail*m
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+ * since m is an integer, the dominant term is exact.
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+ * m has at most 10 digits (for subnormal numbers),
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+ * and log2_hi has 11 trailing zero bits.
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+ *
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+ * log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h
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+ * logF_hi[] + 512 is exact.
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+ *
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+ * log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ...
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+ * the leading term is calculated to extra precision in two
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+ * parts, the larger of which adds exactly to the dominant
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+ * m and F terms.
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+ * There are two cases:
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+ * 1. when m, j are non-zero (m | j), use absolute
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+ * precision for the leading term.
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+ * 2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1).
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+ * In this case, use a relative precision of 24 bits.
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+ * (This is done differently in the original paper)
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+ *
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+ * Special cases:
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+ * 0 return signalling -Inf
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+ * neg return signalling NaN
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+ * +Inf return +Inf
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+*/
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+
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+#define N 128
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+
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+/* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128.
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+ * Used for generation of extend precision logarithms.
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+ * The constant 35184372088832 is 2^45, so the divide is exact.
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+ * It ensures correct reading of logF_head, even for inaccurate
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+ * decimal-to-binary conversion routines. (Everybody gets the
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+ * right answer for integers less than 2^53.)
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+ * Values for log(F) were generated using error < 10^-57 absolute
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+ * with the bc -l package.
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+*/
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+static double A1 = .08333333333333178827;
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+static double A2 = .01250000000377174923;
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+static double A3 = .002232139987919447809;
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+static double A4 = .0004348877777076145742;
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+
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+static double logF_head[N+1] = {
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+ 0.,
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+ .007782140442060381246,
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+ .015504186535963526694,
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+ .023167059281547608406,
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+ .082443669210988446138,
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+ .089612158689760690322,
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+ .096729626458454731618,
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+ .103796793681567578460,
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+ .145182009844575077295,
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+ .158605030176659056451,
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+ .611801541106615331955,
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+ .616029877215623855590,
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+ .620240409751204424537,
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+ .624433288012369303032,
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+ .628608659422752680256,
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+ .641031179420679109171,
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+ .645137961373620782978,
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+ .649227946625615004450,
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+ .653301272011958644725,
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+ .693147180560117703862
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+};
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+
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+static double logF_tail[N+1] = {
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+ 0.,
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+ -.00000000000000543229938420049,
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+ .00000000000000172745674997061,
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+};
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+
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+#if 0
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+double
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+#ifdef _ANSI_SOURCE
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+log(double x)
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+#else
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+log(x) double x;
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+#endif
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+{
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+ int m, j;
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+ double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0;
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+ volatile double u1;
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+
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+ /* Catch special cases */
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+ if (x <= 0)
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+ if (x == zero) /* log(0) = -Inf */
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+ return (-one/zero);
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+ else /* log(neg) = NaN */
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+ return (zero/zero);
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+ else if (!finite(x))
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+ return (x+x); /* x = NaN, Inf */
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+
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+ /* Argument reduction: 1 <= g < 2; x/2^m = g; */
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+ /* y = F*(1 + f/F) for |f| <= 2^-8 */
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+
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+ m = logb(x);
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+ g = ldexp(x, -m);
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+ if (m == -1022) {
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+ j = logb(g), m += j;
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+ g = ldexp(g, -j);
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+ }
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+ j = N*(g-1) + .5;
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+ F = (1.0/N) * j + 1; /* F*128 is an integer in [128, 512] */
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+ f = g - F;
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+
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+ /* Approximate expansion for log(1+f/F) ~= u + q */
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+ g = 1/(2*F+f);
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+ u = 2*f*g;
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+ v = u*u;
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+ q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
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+
|
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+ /* case 1: u1 = u rounded to 2^-43 absolute. Since u < 2^-8,
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+ * u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits.
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+ * It also adds exactly to |m*log2_hi + log_F_head[j] | < 750
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+ */
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+ if (m | j)
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+ u1 = u + 513, u1 -= 513;
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+
|
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+ /* case 2: |1-x| < 1/256. The m- and j- dependent terms are zero;
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+ * u1 = u to 24 bits.
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+ */
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+ else
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+ u1 = u, TRUNC(u1);
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+ u2 = (2.0*(f - F*u1) - u1*f) * g;
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+ /* u1 + u2 = 2f/(2F+f) to extra precision. */
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+
|
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+ /* log(x) = log(2^m*F*(1+f/F)) = */
|
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+ /* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q); */
|
|
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+ /* (exact) + (tiny) */
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|
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+
|
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+ u1 += m*logF_head[N] + logF_head[j]; /* exact */
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+ u2 = (u2 + logF_tail[j]) + q; /* tiny */
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|
|
+ u2 += logF_tail[N]*m;
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|
+ return (u1 + u2);
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|
|
+}
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|
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+#endif
|
|
|
+
|
|
|
+/*
|
|
|
+ * Extra precision variant, returning struct {double a, b;};
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|
|
+ * log(x) = a+b to 63 bits, with a rounded to 26 bits.
|
|
|
+ */
|
|
|
+struct Double
|
|
|
+#ifdef _ANSI_SOURCE
|
|
|
+__log__D(double x)
|
|
|
+#else
|
|
|
+__log__D(x) double x;
|
|
|
+#endif
|
|
|
+{
|
|
|
+ int m, j;
|
|
|
+ double F, f, g, q, u, v, u2;
|
|
|
+ volatile double u1;
|
|
|
+ struct Double r;
|
|
|
+
|
|
|
+ /* Argument reduction: 1 <= g < 2; x/2^m = g; */
|
|
|
+ /* y = F*(1 + f/F) for |f| <= 2^-8 */
|
|
|
+
|
|
|
+ m = logb(x);
|
|
|
+ g = ldexp(x, -m);
|
|
|
+ if (m == -1022) {
|
|
|
+ j = logb(g), m += j;
|
|
|
+ g = ldexp(g, -j);
|
|
|
+ }
|
|
|
+ j = N*(g-1) + .5;
|
|
|
+ F = (1.0/N) * j + 1;
|
|
|
+ f = g - F;
|
|
|
+
|
|
|
+ g = 1/(2*F+f);
|
|
|
+ u = 2*f*g;
|
|
|
+ v = u*u;
|
|
|
+ q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
|
|
|
+ if (m | j)
|
|
|
+ u1 = u + 513, u1 -= 513;
|
|
|
+ else
|
|
|
+ u1 = u, TRUNC(u1);
|
|
|
+ u2 = (2.0*(f - F*u1) - u1*f) * g;
|
|
|
+
|
|
|
+ u1 += m*logF_head[N] + logF_head[j];
|
|
|
+
|
|
|
+ u2 += logF_tail[j]; u2 += q;
|
|
|
+ u2 += logF_tail[N]*m;
|
|
|
+ r.a = u1 + u2; /* Only difference is here */
|
|
|
+ TRUNC(r.a);
|
|
|
+ r.b = (u1 - r.a) + u2;
|
|
|
+ return (r);
|
|
|
+}
|